Site-Specific Oxidation State Assignments of the Iron Atoms in the [4Fe:4S](2+/1+/0) States of the Nitrogenase Fe-Protein

摘要

The nitrogenase iron protein (Fe-protein) contains an unusual [4Fe:4S] iron-sulphur cluster that is stable in three oxidation states: 2+, 1+, and 0. Here, we use spatially resolved anomalous dispersion (SpReAD) refinement to determine oxidation assignments for the individual irons for each state. Additionally, we report the 1.13-angstrom resolution structure for the ADP bound Fe-protein, the highest resolution Fe-protein structure presently determined. In the dithionitereduced [4Fe:4S](1+) state, our analysis identifies a solvent exposed delocalized Fe2.5+ pair and a buried Fe2+ pair. We propose that ATP binding by the Fe-protein promotes an internal redox rearrangement such that the solvent-exposed Fe pair becomes reduced, thereby facilitating electron transfer to the nitrogenase molybdenum iron-protein. In the [4Fe:4S](0) and [4Fe:4S](2+ )states, the SpReAD analysis supports oxidation states assignments for all irons in these clusters of Fe2+ and valence delocalized Fe2.5+, respectively.